Vitamin D has been associated with many health benefits -- improved bone density, fewer fractures, and possibly even reduced risk of breast, colon and prostate cancer -- so it is no surprise that vitamin D supplements have been flying off drug store shelves. However, less attention has been given to the possible safety risks that may be associated with high vitamin D intake. One possible adverse effect of vitamin D is an increased risk of kidney stones, but evidence of a link between vitamin D and kidney stones is inconclusive.
What Are Kidney Stones?
The kidneys are responsible for removing waste substances, like excess calcium, from the body and excreting them through the urine. When there is too much waste or too little water in the urine, these waste substances can begin to crystallize and form a kidney stone. According to the National Kidney Foundation, as many as one in 10 people will have a kidney stone during their lives.
What Does Vitamin D Do?
Vitamin D is important for calcium metabolism. The active form of vitamin D helps make proteins that aid in absorbing calcium from the diet. When calcium is absorbed, some of it is used for cellular activities, some is deposited in your bones, and whatever is left over is excreted in urine. This has led to concern that excess vitamin D could increase urinary calcium too much, creating an increased risk of kidney stones.
Does Vitamin D Increase Risk of Kidney Stones?
Some studies have shown that vitamin D intake is not associated with increased risk of kidney stones. One large study, the Health Professionals Follow-up Study, followed approximately 50,000 men for 14 years. In a 2004 article in the "Journal of the American Society of Nephrology," researchers reported that among the 1,473 men who experienced kidney stones during the follow-up, there was no difference in vitamin D intake compared to men who did not develop kidney stones. A 2009 study in the "British Journal of Urology International" investigated whether vitamin D increases the amount of calcium excreted in urine. The authors gave a large dose of vitamin D, 50,000 international units, or IUs, to women with low vitamin D status for 15 days. While the vitamin D supplementation increased calcium absorption, the women did not have increased calcium in their urine, suggesting that the vitamin D supplementation did not put them at increased risk for kidney stones. In contrast, the National Institutes of Health-funded Women's Health Initiative, or WHI, study, which followed more than 36,000 women assigned to take a calcium and vitamin D supplement or a placebo, found an increased risk of kidney stones with calcium and vitamin D supplementation. In an article appearing in the "New England Journal of Medicine" in 2006, WHI researchers reported a 17 percent increase in kidney stones among the women taking the supplements.
Is Vitamin D Safe?
In November 2010, the Institute of Medicine raised the tolerable upper level of intake for vitamin D to 4,000 IUs per day. This means that the level of vitamin D in a typical supplement, usually 500 to 1,000 IUs, should be safe for most people. Depending on age and gender, the recommended dietary allowance, or RDA, for vitamin D ranges from 600 to 800 IUs per day. Based on current evidence, intakes above this level don't provide any added health benefits, so there is no need to take higher doses that may increase the risk of side effects like kidney stones. Consult your doctor before taking a vitamin supplement, especially if you have a history of kidney stones.
References
- National Kidney Foundation: Kidney Stones
- "Journal of the American Society of Nephrology"; Dietary Factors and the Risk of Incident Kidney Stones in Men: New Insights after 14 Years of Follow-up; Eric N. Taylor, et al.; December 2004
- "British Journal of Urology International"; Vitamin D Repletion Does Not Alter Urinary Calcium Excretion in Healthy Postmenopausal Women; Kristina L. Penniston, et al.; April 2009
- "The New England Journal of Medicine"; Calcium Plus Vitamin D Supplementation and the Risk of Fractures; Rebecca D. Jackson, et al.; February 2006
- Institute of Medicine: Dietary Reference Intakes for Calcium and Vitamin D
